President Reagan, who proposed last week to eliminate federal programs ranging from Amtrak passenger rail subsidies to the Small Business Administration, said last night that he is only trimming "fat" and is not cutting "essential parts" of the government.

In his nationally televised news conference, Reagan suggested that the domestic spending cuts he proposed are not that severe, reducing spending in this area by only 5 percent. He again promised to veto any tax increase "on arrival."

Reagan defended Vice President Bush, who has recently been making the rounds of conservative groups in the intensified jockeying for the the 1988 Republican presidential nomination. Asked whether Bush had been "consistent" with his recent comments on conservative issues, Reagan said, "Well, you're asking a fellow who was once a liberal New Deal Democrat before he became a Republican. Sometimes we do change our minds with things that have gone on."

Reagan could not, in response to a question, cite specific examples in which Bush had made a major contribution to administration policy, but said Bush had been a supporter of "everything we're trying to do."

Questioned about the timing of this year's planned summit with Soviet leader Mikhail Gorbachev, Reagan said the Soviets had not yet come up with a date for the meeting, which has been in doubt for some weeks. "I'd like to have it pinned down," said Reagan, who has proposed a meeting in June or July. He reiterated that the United States would prefer to avoid a September date because it would interfere with the midterm congressional elections.

Asked whether Gorbachev may be seeking to use the timing of the summit to elicit concessions on arms reductions from the United States, Reagan said, "That kind of linkage wouldn't work."

In his opening statement, Reagan defended the fiscal priorities he submitted to Congress last week, which have come under bipartisan criticism. "Those who say that our budget is dead on arrival are really saying, brace yourself for a tax increase," Reagan said. " . . . Rest assured that any tax increase sent to me will be VOA, veto on arrival."

Reagan insisted that Congress meet the deficit targets set in the new Gramm-Rudman-Hollings balanced-budget law. He said the cuts he proposed in fiscal 1987 domestic spending would be made "in the area of fat that's in administration . . . ," and added that no level of government has a higher cost of administration than the federal government. Reagan said, "Now, if we can eliminate some fat, we can handle this."

However, Reagan's budget goes far beyond trimming expenses for administration and proposes the elimination of some programs, such as Amtrak passenger subsidies and the Interstate Commerce Commission, and the transfer of others to states and private business, such as the Naval Petroleum Reserve and the regional power authorities. Reagan is also seeking major reductions in such programs as Medicare and college student aid.

Reagan was asked several questions about domestic policy issues he said he couldn't answer: whether he would be willing to open the Social Security system and Medicare to pay for nursing home care for the chronically ill elderly; whether corporate mergers are getting out of hand; whether budget cuts would hurt veterans who have certificates to purchase Veterans Administration housing.

The president, questioned about the elimination of a job-taining program in his fiscal 1987 budget, responded by recounting how, as California governor, he had set up a welfare reform requiring able-bodied recipients to report to work on public service jobs.

Reagan was also asked about U.S. naval operations north of Libya in the Mediterranean, and said, "I don't know the nature of the operations that have been conducted" there. But Reagan said he ordered some U.S. ships to cross into what he described as exaggerated Libyan claims of territorial waters, "but not getting into what are actually their waters."

"I gave the go-ahead on that and I would again," he said.